Other villains are equally worthwhile, but Heath Ledger's performance was above and beyond. It's a matter of becoming the role. The best examples I can think of is your standard episode of Saturday Night Live: you can definitely tell which hosts are phoning it in, and which hosts are invested in their roles. It's not "letting loose", its becoming the character.

I do think though, that the Joker is such an outlandish character that he could be played by a lot of people and impress an audience just as well as Ledger did.

tighr wrote:

For another example, I've never thought Christian Bale ever truly owned the Batman role, rather he just did a decent job of portraying it.

I agree. As much as I love Nolan's trilogy, Bale never struck me as the ideal embodiment of Bruce Wayne/Batman. He did do an excellent job of playing up the lazy playboy image of Bruce though.

tighr wrote:

Robert Downey Jr, however, IS Tony Stark.

Funny, because he's pretty much "TINO". Tony In Name Only. I don't know everything about Iron Man/Tony Stark, but the guy we saw in the movies isn't the one I've seen in the past. Downey plays his own version of the character, but people like his take because he's a charismatic actor.